This invention pertains generally to gas turbine engines and, more particularly, to the support and cooling of the turbine rotor shroud portion thereof.
Gas turbine engines are made to operate more efficiently by increasing the turbine operating temperatures to very high levels. Since the preferred temperatures are well above the temperatures allowable for use with current flow path metals, it is necessary to provide cooling of these parts in order that they may exhibit acceptable life characteristics. The turbine blades, which operate in the main gas flow stream, are normally cooled by way of convection, impingement or film cooling, or a combination of the three forms. The shrouds, which surround the row of turbine blades forming a stationary outer flow path, are more commonly cooled by impinging a supply of cooling air, as, for example, bleed air from the compressor, to flow directly on the outer surface of the shroud element. Traditionally, impingement of air against the outer surface of the shroud is accomplished by way of an impingement baffle which is mounted to the outer surface of the shroud structure in such a way that the baffle, or the plurality of circumferential baffle segments, form a common boundary between the radially inward low pressure plenum where the impinged air resides, and the radially outer high pressure plenum which is defined in part by the shroud support and which receives the relatively high pressure air from the compressor bleed air manifold or the like. In some installations it has been estimated that the amount of leakage air in such a system is in the order of 40 percent of the total metered shroud cooling airflow. This leakage occurs in any one of a number of leakage paths. Because of the necessity to have a number of interfitting parts such as, for example, the shroud support grooves and the shroud flanges which fit into those grooves, there is a tendency for the high pressure cooling air to leak from the plenum without passing through the impingement baffle. Also, the shrouds are segmented so that the thermal response of the shroud assembly is controlled by the shroud support and, since the shrouds are sized in equal circumferential length such that thermal stress which produces segment straightening is held to a minimum during engine running, there is necessarily considerable leakage between the shroud segments. Higher pressures, of course, tend to increase the cooling effectiveness of the system, but on the other hand tend to also increase the leakage. There is a definite plenum pressure which will minimize the shroud metal temperature. Since the heat extracted from the shroud is a function of the impingement flow times the impingement air heat pickup (cooling effectiveness), there is a plenum pressure where the improved cooling effectiveness of higher plenum pressure is not sufficient to offset the reduction in impingement flow.
It is therefore an object of the present invention to provide an improved shroud support and cooling apparatus.
Another object of the present invention is to provide a shroud cooling apparatus with increased efficiency characteristics.
Yet another object of the present invention is the provision in a shroud cooling apparatus for reduction in high pressure air leakage.
Yet another object of the present invention is the provision in a shroud cooling apparatus for the use of higher pressure air with decreased amounts of leakage.
Still another object of the present invention is the provision in a shroud cooling apparatus for reducing the number of leakage paths from the high pressure side of the impingement baffle.
Yet another object of the present invention is the provision for a turbine shroud support and cooling apparatus which is economical to manufacture and efficient in use.
Another object of the present invention is the provision for a greater percentage of the metered flow to pass out into the flow path through film holes rather than as leakage.
These objects and other features and advantages become more readily apparent upon reference to the following description when taken in conjunction with the appended drawings.